Grain kernels, such as wheat kernels, generally include an inner seed of endosperm surrounded by outer layers of bran. Also within the outer bran layers, adjacent and smaller than the endosperm, is a wheat germ. Conventional commercial milling involves the steps of cleaning the wheat kernels, tempering for approximately 6 to 24 hours, and typically 18 to 24 hours, to soften and increase the moisture content of the wheat kernels, and then passing the kernels through a milling or grinding process including a sequence of counter-rotating mill rollers, also referred to herein as roll stands or roll sets, that separate the outer bran layers and germ from the endosperm through a scraping or abrading action. A roll set is typically operated with a faster roller and a slower roller providing an action whereby a wheat kernel tends to be held against the slow roll while the kernel is scraped by the fast roll. Each roll set produces product and also some small particulates referred to as fines. A typical flour mill will include a sequence of roll sets and sorters such as sifters which provide differing grades of flour and as designed, progressively separate the outer bran layer from the inner endosperm, which endosperm is ground into flour. The output from each set of rollers is sorted into multiple streams, typically based on particle size and density differences, to separate the endosperm from the bran and germ, and to direct higher and lower mineral-containing product to selected roller sets.
The desired milled flour is granular endosperm with some granular germ, with as little as possible of bran and mineral content in the flour. Germ can be an additional product. Conventional dry milling is described in Chapter 6, pages 125-145 of Principles of Cereal Science and Technology, Second Edition, R. Carl Hoseney (American Association of Cereal Chemists. Inc., 1994), hereby incorporated by reference.
It is generally preferred during the grinding process, particularly at the front end of a flour mill, that the bran layers be retained in sections as large as possible to facilitate the segregation of the bran from the desired endosperm. If the grain kernels are not tempered with a fluid such as water before passing through the rollers, the kernels will shatter when passed through the rollers. That shattering breaks the bran into small segments that are more difficult to separate from the desired flour. Tempering of wheat kernels is well known in the art and means the water addition and the corresponding resting time for the water to penetrate the wheat kernel. The tempering conditions or toughens the bran, providing it with a more flexible like texture that is resistant to the shattering. Tempering also helps to establish desired moisture content prior to entry into the roller sets, which assists in controlling the fine particulate flour material. Tempering also imparts characteristics to the endosperm that tend to make it more readily separable from the bran layers, and contributes attributes, such as moisture content, that assist in the production of better quality flour. During tempering the endosperm swells, loses its glassiness appearance and grows sodden. Long tempering periods, in the 6 to 24 hour range, and often in the 18 to 24 hour range, are typically needed as it takes time for the moisture to dampen the inner sections of the kernel. It also appears that the dampening enters in particular through the portion of the kernel where the germ resides. The dampening appears to migrate from the outside of the kernel that is in direct contact with the water, toward the inside of the kernel, which takes time, with most of the dampening entering through the germ.
Although tempering in commercial scale systems typically encompasses an 18 to 24 time period, for quick-turnaround testing purposes, an experimental micromilling method discloses a double fifteen minute tempering (Cereal Chemistry, Volume 62, No. 6, 1985, pages 454-458, Experimental Micromilling: Reduction of Tempering Time of Wheat from 18-24 Hours to 30 Minutes, K. F. Finney and L. C. Bolte). The article teaches processing of 50 gram or 100 gram samples of wheat through a pretemper of 2% moisture for 15 minutes, followed by a prebreak through a Tag-Heppenstall moisture meter roll set and then tempered to 15% moisture for 15 minutes. This process allows determining the quality of the small wheat sample in 50 to 60 minutes, consistent with an about 1-hour time period for loadings of wheat for export shipping.
The time and capacity throughput associated with tempering in a commercial facility adds to the equipment and time requirements of a mill for the production of flour. A longer processing time translates to increased costs for a given output of flour. For a given output of flour the tempering time impacts, for example, the time of plant personnel, holding tank capacities, on-site storage bins, and on-site grain transporting equipment requirements. Subsequent to tempering the wheat is milled in a sequence of roll sets and sifting equipment into flour of varying grades. The more bran that is released as small particles, the more difficult it is to remove the bran and form flour of low mineral content.
Wheat flour is milled throughout the world in large quantities. An improvement in plant capital or operating costs, or improved production efficiency, will be beneficial.